Deforming the Earth’s Crust Chapter 4 Section 4 Vocabulary: 1. compression 2. tension 3. folding 4. fault 5. uplift 6. subsidence

Deformation p. 112 Different things happen to rock when different types of stress are applied. Stress is the amount of force per unit area on a given material. The process by which the shape of a rock changes because of stress is called deformation. Rock layers bend when stress is placed on them. When enough stress is placed on rocks, they can reach their elastic limit and break. The type of stress that occurs when an object is squeezed, such as when two tectonic plates collide, is called compression. When compression occurs at a convergent boundary, large mountain ranges form. Another form of stress is tension. Tension is stress that occurs when forces act to stretch an object. This occurs at divergent boundaries, such a mid-ocean ridges, when two tectonic plates pull away from each other.

The bending of rock layers because of stress in the Earth’s Folding p. 113 The bending of rock layers because of stress in the Earth’s crust is called folding. Depending of how the rock layers deform, different types of folds are made. Most common folds Rock layers fold so both of the fold are horizontal. Folds can be large or small. The largest folds are measured in kilometers. Other folds are also obvious, but much smaller. These small folds can be measured in centimeters.

Some rock layers break when stress is applied to them. The Faulting p. 114 Some rock layers break when stress is applied to them. The surface along which rocks break and slide past each other is called a fault. The blocks of crust on each side of the fault are called fault blocks. A normal fault is shown to the right. When a normal fault moves, it causes the hanging wall to move down relative to the footwall. This usually happens when tectonic forces pull rocks apart. Hanging wall Footwall A reverse fault is shown to the left. When a reverse fault moves, it causes the hanging wall to move up relative to the footwall. This usually happens when tectonic forces push rocks together. Hanging wall Footwall

A third major type of fault is called a strike-slip fault. Strike-slip faults form when opposing forces cause rock to break and move horizontally. If you were standing on one side of a strike-slip fault looking across the fault when it moved, the ground on the other side would appear to move to your left or right. The San Andreas Fault in California is a spectacular example of a strike-slip fault. When rocks are moved horizontally by opposing forces, strike-slip faults often form.

Plate Tectonics and Mountain Building p. 116 Mountains exist because tectonic plates are continually moving around and colliding with one another. When tectonic plates undergo compression or tension, they can form mountains in several ways. Three of the most common types of mountains are folded mountains, fault-block mountains, and volcanic mountains. Folded Mountains The highest mountain ranges in the world are made up of folded mountains. They form at convergent boundaries when continents have collided. Folded mountains form when rock layers are squeezed together and pushed upward. About 390 million years ago, the Appalachian Mountains formed when the landmasses that are now North America and Africa collided at a convergent boundary. The Appalachian Mountains were once as tall as the Himalaya Mountains but have been worn down by hundreds of millions of years of weathering and erosion.

Fault-Block Mountains Fault-block mountains form when tension causes large blocks of Earth’s crust to drop down relative to other blocks. When sedimentary rock layers are tilted up by faulting they can produce mountains that have sharp, jagged peaks. The Tetons formed as a result of tectonic forces that stretched the Earth’s crust and caused it to break in a series of normal faults, creating this fault-block mountain range. Volcanic Mountains Most of the world’s major volcanic mountains are located at convergent boundaries where oceanic crust sinks into the asthenosphere at subduction zones. The rock that is melted in subduction zones forms magma, which rises to the Earth’s surface and erupts to form volcanic mountains. Volcanic mountains can also form under the sea. They can also rise above the ocean to become islands. The majority of volcanic mountains have formed around the tectonically active rim of the Pacific Ocean known as the Ring of Fire.

Uplift and Subsidence p. 118 The rising of regions of Earth’s crust to higher elevations is called uplift. The sinking of regions of Earth’s crust to lower elevations is known as subsidence. Unlike some uplifted rocks, rocks that subside do not undergo much deformation. The formation of mountains is one type of uplift, but uplift can also occur when large areas rise without deforming. One way areas rise without deforming is called rebound. When the crust rebounds, it slowly springs back to its previous elevation when a weight is removed from the crust. Hot rocks take up more space than cooler rocks. The farther the lithosphere is from hot rocks the cooler and denser the lithosphere becomes. Because the lithosphere takes up less space, the ocean floor subsides. Subsidence can also occur when the lithosphere becomes stretched in rift zones. A rift zone is set of deep cracks that forms between two tectonic plates that are pulling away from each other. As tectonic plates pull apart, stress between the plates caused a series of faults to form along the rift zone.

Quiz Time! 1. Explain compression and tension. Compression can cause rocks to be pushed into mountain ranges as tectonic plates collide at convergent boundaries. Tension can pull rocks apart as tectonic plates separate at divergent boundaries. 2. How do folded mountains form? Folded mountains form when rock layers are squeezed together and pushed upward. 3. What three features form when rock layers bend? Anticlines, synclines, and monoclines

4. The type of fault in which the hanging wall moves up relative to the footwall is called a A. strike-slip fault B. fault-block fault C. normal fault D. reverse fault D 5. What are the most common kind of mountains? folded mountains, fault-block mountains, and volcanic mountains. Science Humor… What did one mountain say to the other mountain? Let’s meet in the valley!